Boba Fett was still alive when hell into the Sarlacc, which are specifically told keeps it's food alive for a thousand years. So we knew he was more or less alive, it was just a question of whether or not he got out of the Sarlacc, which we now know he did.
Well, no, it says that the Sarlaac slowly "digests" its prey over 1000 years. That just means it takes a long time to make it through the Sarlaac's stomach and intestines, not that you're alive while all that's happening. Frankly, IMO, Fett should have stayed dead. Bringing him back was just the worst kind of fan service.
Though I agree with your larger point that Mace is definitely dead as disco.
He didn't have to have gotten a new teacher, we saw in TLJ that he had a whole collection of old Jedi texts, so he could have easily learned all of that through those books, without ever running into another Jedi.
True, but one shouldn't even need books. Luke is a Jedi, he had 30 odd years to train himself and get more powerful. You don't need a teacher for that, especially since once Jedi become knights, they are no longer taught by masters, they're on their own.